In a cellular network for radio communication, a Time Division Duplex (TDD) configuration of subframes may be used for uplink and downlink transmissions in cells where consecutive subframes are comprised in a repeatable radio frame. The subframes are reserved for uplink transmissions from User Equipments (UEs) to a serving base station and for downlink transmissions from the base station to the UEs such that uplink and downlink transmissions do not occur at the same time within the cell. A subframe is basically defined by a preset time period of a certain length, typically 1 millisecond (ms), and each subframe may comprise two time slots of 0.5 ms each. Further, a radio frame comprises a predefined number of consecutive subframes, e.g. ten subframes. In such a network, different base stations are able to use different configurations of subframes, e.g. depending on the current need for uplink and downlink bandwidth resources.
An example of different TDD configurations that can be used by base stations in different cells is shown in the table of FIG. 1 comprising seven different TDD configurations 0-6 each having ten subframes 0-9 including downlink subframes “D”, uplink subframes “U” and so-called special subframes “5”. The special subframes S are configured with one part reserved for downlink, another part reserved for uplink, and a guard period between the two parts allowing neither uplink nor downlink. It can be seen in this example that the first three subframes 0-2 and subframe 5 are reserved for downlink D, special S, uplink U, and downlink D, respectively, in all TDD configurations 0-6, while the remaining subframes 3, 4, 6-9 can vary in different TDD configurations. The latter subframes 3, 4, 6-9 may thus be referred to as flexible subframes.
In this disclosure, the term “flexible subframe” denotes a subframe in which the direction of transmission, i.e. downlink or uplink, may differ between different cells so that the flexible subframe is used for downlink in one cell and for uplink in another cell. Further, a flexible subframe may differ from one radio frame to another in the same cell so that the flexible subframe is used in the cell for downlink in one radio frame and for uplink in another radio frame. Thereby, transmissions in flexible subframes may, at least in some radio frames, cause interference between different neighboring cells as follows. It should be noted that a flexible subframe may also be a special subframe used partly for uplink transmissions and/or partly for downlink transmissions. In this disclosure, the expression “during a flexible subframe” should be understood as in the flexible subframe or in a subframe that overlaps in time with the flexible subframe depending on if the UE is served by the base station that applies the subframe scheme with the flexible subframe or if the UE is served by a neighboring base station that applies a subframe scheme with a subframe that overlaps or coincides in time with the flexible subframe.
When different TDD configurations are used in two neighboring cells, interference may occur across the cells during a flexible subframe where downlink is permitted in one cell and uplink is permitted in the other cell at the same time. In this description, the term “neighboring cells” means that they are close enough to one another so that transmissions in one cell can potentially cause interference in the other cell. Interference due to different TDD configurations in neighboring cells can be either 1) downlink to uplink interference when a downlink transmission from a base station of one cell disturbs an uplink reception in a base station of the other cell during a flexible subframe, or 2) uplink to downlink interference when an uplink transmission from a UE in one cell disturbs a downlink reception in a UE in the other cell during a flexible subframe. Of these two scenarios, 1) refers to interference between base stations which is more or less predictable and this interference can be controlled quite accurately since the base stations in the neighboring cells are in fixed positions relatively far away from each other such that downlink signals from one base station are not very strong when received in the other base station.
On the other hand, scenario 2) above refers to interference between UEs which is more unpredictable since the UEs move around and may sometimes be located quite near each other while being served by different base stations, e.g. when both are located close to the borders of their respective cells. This scenario is illustrated in FIG. 2 where a first UE denoted “UE1” is located near the border of a first cell 1 served by a first base station “BS1” using a TDD configuration allowing the UE1 to transmit uplink signals “UL1” in a certain subframe. At the same time, a second UE “UE2” is located near the border of a second cell 2 served by a second base station “BS2” using another TDD configuration allowing UE2 to receive downlink signals “DL2” in the same subframe, thus being a flexible subframe in this context. Since UE1 and UE2 are quite close to one another but relatively far away from their respective base stations, the uplink signals UL1 transmitted with high power from UE1 will interfere strongly with the relatively weak downlink signals DL2 received by UE2 during the flexible subframe. This UE to UE interference “I” is illustrated by a dashed arrow.
FIG. 3 shows two examples of TDD configurations which can cause UE to UE interference across neighboring cells 1 and 2. In cell 1, TDD configuration 1 of FIG. 1 is used and in cell 2, TDD configuration 2 of FIG. 1 is used. It can be seen in both of FIG. 1 and FIG. 3 that flexible subframes 3 and 8 are configured differently in the two cells such that they are uplink subframes in cell 1 and downlink subframes in cell 2, hence potentially causing UE to UE interference I from cell 1 to cell 2, illustrated by dashed arrows in FIG. 3. In this case, UE1 can be called an “aggressor UE” and UE2 can be called a “victim UE”. Likewise, cells 1 and 2 can be called “aggressor cell” and “victim cell”, respectively. It is thus a problem that, in a radio communication network allowing different TDD configurations with one or more flexible subframes in different cells, downlink radio signals received by a victim UE in a victim cell during a flexible subframe, may be subjected to interference caused by an uplink transmission from an aggressor UE in an aggressor cell during that subframe, e.g. depending on the relative distance and locations of the UEs which are typically unpredictable.